HERFD-XANES and XES as complementaryoperandotools for monitoring the structure of Cu-based zeolite catalysts during NO_x-removal by ammonia SCR

Abstract

Abstract. In this article, we demonstrate the potential of hard X-ray techniques to characterize catalysts under working conditions. Operando high energy resolution fluorescence detected (HERFD) XANES and valence to core (vtc) X-ray emission spectroscopy (XES) have been used in a spatially-resolved manner to study Cu-zeolite catalysts during the standard-SCR reaction and related model conditions. The results show a gradient in Cu oxidation state and coordination along the catalyst bed as the reactants are consumed. Vtc-XES gives complementary information on the direct adsorption of ammonia at the Cu sites. The structural information on the catalyst shows the suitability of X-ray techniques to understand catalytic reactions and to facilitate catalyst optimization. IntroductionOver the past years, X-ray absorption spectroscopy (XAS) has been successfully applied to elucidate the structure of metal sites in a variety of solid compounds and liquid phase complexes. The application of high energy resolution fluorescence detected -X-ray Absorption Near Edge Structure (HERFD-XANES) spectroscopy offers the possibility to unravel additional spectral features and thus more information on the sample compared to conventional XAS [1,2]. This high resolution is achieved by measuring the fluorescence signal of the Kß-line (3p  1s). The 3p orbitals interact more with the valence 3d and 4p orbitals and therefore show a stronger influence on the environment than the less pronounced mixing of 2p and 3d orbitals, which results in the Kα-line [3]. By selecting a Kß-line as a single fluorescence channel, a higher energy resolution is achieved as the core-hole life-time during the X-ray absorption process can be circumvented [4]. An additional technique to characterize metal sites is X-ray emission spectroscopy (XES), where electrons from an occupied higher orbital are transferred to an empty lower orbital. Information on occupied states is therefore gained by XES, whereas XANES is a long established complementary technique that provides information on the electron transfer to unoccupied states [5]. Particularly the transfer of electrons from valence orbitals provides new insights into the interaction between metal

Activity of Cu–Al–Oxo Extra-Framework Clusters for Selective Methane Oxidation on Cu-Exchanged Zeolites

“…The combined use of K- and L 3 -edge XANES allows unambiguous determination of the coordination structure and environment of Cu. Using HERFD-XANES resolves spectral features in the Cu K edge (Figure a) that are not detectable using conventional XANES. − However, it must be noted that the peak heights are different between the two measurements. The Cu K-edge XANES spectrum is provided for comparison (Figure S5a) in the SI.…”

Probing the electronic and geometric structures of photoactive electrodeposited Cu2O films by X-ray absorption spectroscopy

“…In this study, we show that high resolution XAS, available at certain synchrotron radiation centralised facilities, can complement laboratory accessible tools in providing insight into the structures of electrodeposited Cu 2 O films and relate it to the activity as photocathodes. High resolution XAS methods are very widely used for powder based catalysts [27,28], but very few studies have been conducted for film-based catalysts [29]. It is striking to find that the presence of small amounts of Cu 2+ species is correlated to the enhanced photocatalytic activity of our electrodeposited Cu 2 O photocathodes, albeit at the expense of the stability.…”

Catalytic Profile Reactor for Multimodal Operando Measurements during Periodic Operation

“…Operando measurement concepts in catalysis combine catalytic activity measurements (e. g. gas and temperature analysis) with structural characterization (e. g., X‐ray diffraction (XRD), [6] X‐ray absorption spectroscopy (XAS), [7] X‐ray emission spectroscopy (XES), [8,9] etc.) during operation, thus in the actual working state of the catalyst.…”

“…during operation, thus in the actual working state of the catalyst. To harvest as much relevant and complementary information as possible, multimodal operando measurement concepts aim to cover broad space and time scales, and combine multiple characterization techniques within the same reactor [9,10] …”

“…The field of operando research evolved to meet this challenge, aiming to unravel catalyst structure-activity relationships through detailed modeling [3,4] and multimodal experimental approaches. [5] Operando measurement concepts in catalysis combine catalytic activity measurements (e. g. gas and temperature analysis) with structural characterization (e. g., X-ray diffraction (XRD), [6] X-ray absorption spectroscopy (XAS), [7] X-ray emission spectroscopy (XES), [8,9] etc.) during operation, thus in the actual working state of the catalyst.…”